Yep, 50W/sf is the generally accepted bell-curve centrepoint for HPS.

Tents of pandafilm within a room work great. They don't necessarily need verticals; a rectangle of 2x4 screwed to the ceiling or joists above, with a sheet of film over the top and the perimeter wrapped with film make a fine, reflective room-within-a-room. Nice technique for basements, attics, garages, etc.